The luminous appearance of a cat's eyes in the dark is due to the reflection of light by the tapetum lucidum, which is part of the membranous layer between the retina and the outer covering of the pupils of the eyes. This remarkable layer is distinctly differentiated from the choroid layer only in certain animals. It is the tapetum lucidum that enables members of the feline family and other nocturnal animals to see even when there is very little light. Some authorities believe this layer reflects the light rays through the retina a second time. In the domestic cat the tapetum lucidum is brilliant green or blue in color and has a metallic luster. The well known glare is especially noticeable when artificial light is thrown on a cat in a dark corner, or when a cat in a dark place is observed through an opening, light entering the darkness through the opening being reflected from the cat's eyes as from a mirror.